This invention relates generally to radio frequency filters, including film bulk acoustic resonators (FBARs).
Film bulk acoustic resonators have many advantages over other techniques, such as surface acoustic wave (SAW) devices and ceramic filters, particularly at high frequencies. For example, SAW filters begin to experience excessive insertion losses above 2.4 gigahertz and ceramic filters are much larger in size and become increasingly difficult to fabricate at increased frequencies.
A conventional FEAR filter may include two sets of FBARs to achieve the desired filter response. The series FBARs have one resonant frequency and the shunt FBARs have another resonant frequency. The frequency of an FBAR is mainly determined by the thickness of its piezoelectric film, which approximately equals a half wavelength of the acoustic wave. The frequencies of FBARs need to be precisely set to achieve a desired filter response.
For example, for a 2 gigahertz FBAR, the thickness of the piezoelectric film may be approximately 2 microns. A one percent error or variation in piezoelectric film thickness may change the frequency of the filter by approximately 20 megahertz, which is not acceptable if a frequency precision or accuracy of 16 megahertz is required.
A variety of techniques are known for adjusting the frequency of an FBAR filter. However, the frequency of an FBAR filter may shift during or after packaging.
Therefore, there may be a need to tune the frequency of FBAR filters that were tuned prior to packaging, again after packaging.